Method of producing bright surfaces on aluminum



Patented May 12, 1936 PATENT OFFICE METHOD OF PRODUCING BRIGHT SURFACES ON ALUMINUM:

Ralph 8. Mason, NewKensington, and Martin Tosterud, Arnold, Pa., assignors to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application February 10, 1934, Serial No. 710,722

11 Claims. ('01. 204-1) This invention relates to bright aluminum surfaces, and it is particularly concerned with a method for the treatment of aluminum articles to produce thereon permanent bright surfaces of high reflectivity.

Bright aluminum surfaces have found extensive application for decorative purposes, and particularly for the reflection of radiant energy of various sorts, such as for light reflection. These bright aluminum surfaces may be produced either by etching for the production of diffusely reflecting surfaces, or by mechanical bufling or polishing to produce specular surfaces. This invention is concerned primarily with a method of increasing and preserving the brightness of such aluminum surfaces, and particularly their light reflecting power.

It is an object of the invention to provide a method .for the treatment of an aluminum surface whereby the reflection factor of such surface is increased, and to provide such a surface with a protective transparent coating. More particularly it is an object of the invention to provide a method for the treatment of an aluminum surface to remove impurities therefrom and to provide such surface with a thin, transparent, protective coating. Other objects of the invention will become apparent from the following description.

Various attempts have been made in the past to provide aluminum reflecting surfaces with transparent protective coatings for the purpose of preserving their reflecting power and making them more durable under severe exposure conditions. Attempts have been made to provide such transparent protective coatings by anodic oxida tion in various electrolytes. In general, however, these attempts have been unsuccessful because the oxide coatings, when produced in sufficient thickness to afford substantial protection, appeared more or less translucent or foggy, and the reflectivity of the treated surface was substantially decreased. Impurities in the aluminum, and dirt, grease or other foreign matter held on the aluminum surface are apparently the cause of this translucence and fogginess in the oxide coating. Furthermore, certain of the oxide coatings when produced are slightly colored, which further detracts from the reflectivity of the treated surface. Anodic coatings produced in chromic acid electrolytes, for example, generally have a yellowish cast and have been, in general, considered unsatisfactory for this purpose.

We have now found that by the addition of hydrofluoric acid to chromic acid electrolytes,

the normal oxidizing action of such electrolytes upon aluminum made anodethereinmay be modifled to produce a combined brightening and coating action, and that'by anodically treating an aluminum article in an electrolyte containing 5 chromic acid and hydrofluoric acid such impurities and foreign matter as may be present on the aluminum surface may be removed and a thin, substantially transparent and colorless protective film simultaneously produced on the surface. 10 There is thus obtained a more durable reflecting surface generally having an increased reflection factor. Furthermore, the bright coated aluminum reflecting surface so obtained may be further treated to increase its durability by anodic 15 oxidation without seriously impairing its reflection factor. Other forms of protective coatings, such as clear lacquer or varnish coatings, may also be applied to the bright, durable reflecting surface by the method of this invention, if desired.

The addition of hydrofluoric acid to the chromic acid electrolyte modifies the normal electrochemical action of the chromic acid solution on the aluminum surface by decreasing its oxide coatforming properties and imparting to the electrolyte the properties of removing impurities present in and on the aluminum surface. This property of removing impurities is apparently due to an increase in the solvent action of the electro-- lyte. This solvent action is, however, so uniform so under the influence of the applied electric current that even when a polished specular aluminum reflecting surface is treated according to the invention as anode in a chromic acid electrolyte 35 containing a small amount of hydrofluoric acid, the surface is cleaned and brightened with an increase in the reflection factor depending upon the amount of impurities and dirt removed from the surface, and at the same time the specular characteristics of the surface are only slightly modified. Hydrofluosilicic acid and other fluorine compounds which yield hydrofluoric acid when in solution with chromic acid, such as the fluoride salts, may be substituted for hydrofluoric acid in 45 equivalent amounts for the purpose of this invention.

The minimum effective amount of hydrofluoric acid necessary to impart to the chromic acid electrolytes the desired properties varies somewhat. Very small amounts of hydrofluoric acid 50 in the chromic acid solution will impart the desired properties in some degree. In general, however, it is considered desirable to have present in the electrolyte not less than about 0.2 per cent of HF by weight. With increased amount of hydrofluoric acid present, the possibility of direct chemical attack on the aluminum surface by the electrolyte is increased, and it is necessary to increase the current density of the applied electric current to prevent this direct chemical attack which tends to produce irregularities in the reflecting surface. While, therefore, higher concentrations of hydrofluoric acid may be used with high current densities, it is generally not desirable that there be present in the electrolyte more than about 1.5 per cent of HF by weight because of the high current consumption required to produce satisfactory results. For most purposes it is preferred to use an electrolyte containing 0.5 to 1.0 per cent HF by weight.

Satisfactory results may be obtained according to this invention by the addition of hydrofluoric acid to chromic acid electrolytes containing from about 1.0 per cent to 25.0 per cent by weight CIOa. While, within this range, the concentration of the chromic acid solution is not critical, it is generally preferred to use electrolytes containing about 5.0 to 10.0 per cent by weight of CrOa.

In carrying out the electrolytic treatment, it is preferable that the temperature of the electrolyte be somewhat above room temperature, and temperatures between about 30 and 70 have proved most satisfactory. With operation of the bath at these temperatures, satisfactory results are generally obtained by a treatment of 5 to 30 minutes. In most cases it is preferred to treat at a temperature of 50 C. for about 10 minutes.

The current density used will vary with the exact composition of the electrolyte and the temperature of operation. The current density should be suflicient to prevent any direct chemical attack of the electrolyte on the aluminum surface, and this current density should not be substantially exceeded. With greater current density, the modifying effect of the hydrofluoric acid on the oxide coat-forming properties of the chromic acid islessened. In general, higher current densities are required for operation at the higher temperatures. When using direct current with electrolytes containing 1 per cent to 25 per cent chromic acid and 0.2 per cent to 1.5 per cent hydrofluoric acid, at temperatures of about 30 to 70 0., satisfactory results are obtainable with current densities in the range of about 20 to 140 amperes per square foot of anode surface. With the lower current densities it is generally desirable to increase the time of treatment.

Prior to the treatment of an aluminum reflecting surface according to this invention it is sometimes desirable, in order to obtain the best results, to preliminarily treat the aluminum surface to remove superficial grease and dirt such as may be present thereon as the result of a previous polishing operation. Any convenient method of preliminary cleaning may be employed, preferably by washing the surface with a solvent which does not objectionably attack the metal and which does not require such rubbing of a polished surface as may tend to injure its appearance.

Subsequent to the electrolytic treatment of the aluminum reflecting surface in the chromic acidhydrofluoric acid electrolyte, the reflecting surface may be anodically' oxidized to form thereon a comparatively dense oxide coating which is substantially transparent. By the term oxide coating as used herein and in the appended claims is meant such coatings concurrently so designated in the art which consist in substantial part of aluminum oxide. It is desirable that the oxide coatings produced be substantially colorless, clear and transparent, in order that the reflectivity of the brightened aluminum surface will be reduced as little as possible. While any electrolyte capable of producing such coatings may be used, it has been found. that sulfuric acid and oxalic acid solutions are particularly suitable i'or this purpose. The amount of reduction in face may be subjected to treatment in water at to C. to make the coating impervious. As a final treatment, and particularly when the step of treating in hot water has been employed, a very light polishing operation with a mildly abrasive material, such as magnesia, silver polish or a mildly abrasive soap powder, may be desirable to remove any superficial deposit which may have been formed on the oxide-coated reflecting surface by reason of any of the previous operations.

The following specific example of a method of carrying out this invention shows the advantages resulting therefrom.

An aluminum article having a high purity aluminum surface was buffed to produce a specular reflecting surface which had a reflection factor of 74.9 per cent. The article was then made the anode in an electrolytic cell containing an electrolyte formed by the addition of 1.0 per cent of hydrofluoric acid containing 48.0 per cent HF to a solution of chromic acid containing 10.0 per cent by weight of CXOs. A direct current at a current density of about 46 to 52 amperes per square foot was employed at a potential of about 16 to 22 volts for 10 minutes, the electrolyte being maintained at a temperature of about 49 to 58 C. The reflecting surface, after this treatment, had a reflection factor of 87.2 per cent. The article was then anodically oxidized in a 7.0 per cent sulfuric acid solution at a temperature of about 25 (3., using a current density of about 12.0 amperes per square foot and a potential of 20 to 22 volts for 10 minutes. The oxide-coated reflecting surface thus obtained was treated with pure boiling water for about 10 minutes and was polished with a mildly abrasive soap powder. The reflection factor of the protected reflecting surface thus obtained was about 85.3 per cent. The article could be handled without permanent marking and staining and could be readily washed or wiped without deterioration of its reflecting power.

In measuring the light reflection factors of the surfaces above referred to, the Taylor reflectometer devised by A. H. Taylor of the Nationa1 Bureau of Standards and described in the Scientific Papers of the Bureau of Standards Nos. S-391 and 405, was used.

In general, aluminum alloy reflecting surfaces, when treated according to the invention in chromic acid-hydrofluoric acid electrolytes and subsequently anodically oxidized, do not have as high reflection factors as are obtainable by the same treatment on a high purity aluminum surface. However, the method herein described is applicable to many aluminum base alloys with advantage, and the term aluminum as used throughout this specification is to be understood to include both aluminum and aluminum base alloys.

We claim:

1. A method of brightening aluminum surfaces and simultaneously producing thereon a transparent, colorless fllm, comprising removing impurities and dirt from said surface by treating said surface anodically in an electrolyte containing chromic acid and hydrofluoric acid.

2. A method of brightening aluminum surfaces and simultaneously producing thereon a transparent, colorless film, comprising removing impurities and dirt from said surface by treating said surface anodically in an electrolyte containing 1.0 to 25.0 per cent by weight of CrOa and 0.2 to 1.5 per cent by weight of HF.

3. A method of producing an aluminum article having a durable, bright surface, comprising anodically treating the aluminum article in an electrolyte containing chromic acid and hydrofluoric acid, and thereafter producing on said surface a clear, transparent, impermeable coating.

4. A method of producing an aluminum article having a durable, bright surface, comprising anodically treating the aluminum article in an electrolyte containing chromic acid and hydrofluoric acid, and thereafter producing on said surface, by anodic oxidation, a clear, transparent coating consisting substantially of aluminum oxide.

5. A method of producing an aluminum article having a durable, bright surface, comprising anodically treating the aluminum article in an electrolyte containing chromic acid and hydrofluoric acid, and thereafter producing on said surface, by anodic oxidation, a clear, transparent coating consisting substantially of .aluminum oxide and impermeabilizing the oxide-coated surface by treating with hot water.

6. A method of producing a specular, highly reflecting surface on aluminum, comprising removing impurities and dirt from a polished aluminum reflecting surface by treating said surface anodically in a solution of chromic acid and hydrofluoric acid at a current density sufflcient to substantially prevent direct chemical attack of the electrolyte on the aluminum surface.

7. A method of producing a durable, specular, reflecting surface on aluminum, comprising treating a polished aluminum reflecting surface anodically in a solution of chromic acid and hydrofluoric acid, and thereafter anodically oxidizing the clean bright surface obtained.

8. A method of producing a durable, specular, reflecting surface on aluminum, comprising treating a polished aluminum reflecting surface anodically in a solution of chromic acid and hydrofluoric acid, and thereafter anodically oxidizing the clean bright surface obtained and treating the mode-coated surface with water at to C.

9. A method of producing an aluminum article having a durable, bright surface, comprising anodically treating the aluminum article in an electrolyte containing 1.0 to 25.0 per cent by weight of CrO: and 0.2 to 1.5 per cent by weight of HF, and thereafter producing on said surface a clear, transparent, impermeable coating.

10. A method of producing an aluminum article having a durable, bright surface, comprising anodically treating the aluminum article in an electrolyte containing 1.0 to 25.0 per cent by weight of CrO: and 0.2 to 1.5 per cent by weight of HF, and thereafter producing on said surface. by anodic oxidation, a clear, transparent coating consisting substantially of aluminum oxide.

11. A method of producing an aluminum article having a durable, bright surface, comprising anodically treating the aluminum article in an electrolyte containing 1.0 to 25.0 per cent by weight of CrOa and 0.2 to 1.5 per cent by weight 

